1. Technical Field
The present invention relates to reduced-moisture-content-solvent ink compositions and methods for producing the ink compositions.
2. Related Art
Nowadays, the ink-jet technology finds numerous applications in printing, one of which is metallic printing. JP-A-2002-179960, for example, discloses a technique for ink-jet printing using an ink composition containing a pigment formed of metal-coated plastic spherical particles. To achieve a highly metallic gloss, however, the spherical particles must be deformed flat so that the surfaces thereof become smooth. To that end, according to this technique, roller pressing and heating must be simultaneously performed. Thus, this technique requires a complicated apparatus and production process and also imposes limitations on recording media.
JP-A-2003-292836 and JP-A-2003-306625, on the other hand, disclose a technique using an ink composition having a colloid of a noble metal such as gold or silver dispersed therein. However, if the particle size is reduced to several to several tens of nanometers to ensure dispersion stability, the ink composition does not provide a metallic gloss because the noble metal colloid shows a color derived from plasmon absorption. In this case, a metallic gloss can be achieved by heating a dried coating at 150° C. or more to fuse the colloid particles. However, although a metallic gloss can be achieved by this technique, it is difficult to achieve a highly metallic specular gloss whose 20°, 60°, and 85° specular glossinesses exceed 200, 200, and 100, respectively, as a uniform surface without irregularities. On the other hand, if the particle size is increased to ensure a metallic gloss, the dispersion stability is decreased, and accordingly aggregation and sedimentation are inevitable, thus significantly decreasing the storage life of the ink composition.
The inventors have proposed a technique disclosed in JP-A-2008-174712 as a pigment dispersion or ink composition for solving the above problems.
The ink composition provided by the technique disclosed in the above publication is superior from the viewpoint of achieving a highly metallic specular gloss. However, a large amount of moisture may be contained in the pigment dispersion containing the metal pigment or the ink composition. In that case, the glossiness of the metal pigment can be decreased by surface corrosion resulting from a chemical reaction such as oxidation or hydroxylation or a chemical reaction promoted by water, that is, a reaction with an oxidizing or acidic gas in air (such as SOx, NOx, or CO2). In addition, if hydrogen gas is generated as a result of the above chemical reaction, it is difficult to preserve the ink composition in an airtight container over an extended period of time. In particular, this tendency is noticeable at elevated temperatures. Thus, the above technique leaves room for improvement.